Set a quitting time. Don't worry about starting your work day, that will happen by itself, but you need to put a good cap on the end of the work day, so you know when to walk away from work.
Have a place to work. I have a home office, but it could be a specific desk, room, or beach chair. Wherever it is, try to do your work there when family is around, that way they know you're working.
Leave your work laptop at your desk. Get a second laptop, tablet, etc that doesn't have work stuff on it if you're going to use electronic stuff around the family outside work hours.
And perhaps most importantly, take advantage of it! I have been able to watch my kids grow up in a way 99% of fathers can't dream of. I take them to dentist appointments, keep an eye on them once in a while when my wife needs to run an errand, and get to see the amazing lego creations they build right away - they don't have to wait till daddy gets home... My wife constantly drops comments like 'it's such a blessing that your job lets you be here for this', and it's true - I wouldn't trade it for the world.